Starting Electronics

Capacitors

Figure

4.10 A

graph showing

an

exponential

curve,

related

to

the

time

constant

You

can check this, if you like, against your curves, or

those in Figures 4.7 and 4.9, where you will find

that

the

times when the capacitor voltage is about 5V7 (that is,

0.63

times the total voltage

change

of 9 V) are about 8

seconds and 4 seconds. Not bad when you consider pos-

sible

experimental errors: the biggest of which is the

existence

of the meter resistance.

89

In a capacitor circuit like

that

of Figure 4.4, the exponen-

tial curve's time constant is given simply by the

product

of

the capacitor and resistor value. So, in the case of the

first